Problem: Evaluate $7r -\dfrac{15}s$ when $r=3$ and $s=5$.
Solution: Let's substitute $ r= 3$ and $ s=5$ into the expression. $\phantom{=}7 {r} -\dfrac{15}{{s}}$ $=7({3})-\dfrac{15}{{5}}$ $=21-3$ $=18$